The invention resides in a centralizing or radially stabilizing bushing that reduces vibration and will maintain contact, alignment and general stability between a shaft and a rider, such as a nut and a threaded lead screw on which the nut translates when the screw rotates. The bushing is so constructed as to maintain these relationships while ignoring temperature changes.
It is important that riders such as a nut or hub that mounts a load, particularly one which is cantilevered, and which rider translate or run along a shaft or a rotating screw be free from radial play or motion transversely of the shaft axis. A typical mechanism is shown in our U.S. Pat. No. 4,566,345.
If, for example, the load were a laser device which aims at a target at a substantial distance from the laser, looseness of the mounting nut relative to the screw in the radial direction, of even a few ten-thousands of an inch, can result in the laser beam missing a target by feet, even though the angle of error is very small.
Conventionally, such nuts and shafts have both been made of steel or other like metals. There are a number of disadvantages of running steel-on-steel. Periodic or continuous lubrication is required, otherwise the metal will be subject to spalling, seizing up or galling. However, in many instances, lubrication must be limited, if not completely prohibited due to the environment in which the mechanism is being operated. Likewise, the product which is being made could be ruined by the lubricant.
Another problem is noise. Often, metal running on metal is noisy when there is a degree of looseness, regardless of lubrication, because of harmonic and other vibrations induced in the metal.
Attempts have been made to overcome the lubrication and noise problems by constructing the bearing of one metal and the shaft of another, the bearing usually being made of a softer metal than the shaft. This solution itself creates problems due to galvanic action and from temperature changes inducing inconsistent thermal expansion or contraction.
A circular bearing grows radially with heat with both its inside and outside diameters expanding. Conversely, cold tends to reduce both the internal and external diameters. This is true, even though both the shaft and the bearing have threads, in which instance the "bearing" would be an internally threaded nut. Consequently, a shaft of one metal or a threaded rod rotating in a circular bearing or threaded nut of another metal may either bind or become loose with temperature changes.
The amount of expansion or contraction of a metal due to heat or cold is a function of its coefficient of thermal expansion expressed in inches, per inch, per degree fahrenheit. If the bearing or nut has a higher coefficient of expansion than the shaft or screw, upon being subjected to heat, looseness resulting in play will be created between the screw and the nut resulting in noise and inaccurate operation. Conversely, if the nut and the screw, or the bearing and shaft, become colder during operation, as for example from an operation moving from indoors to outdoors, if the coefficient of expansion of the nut is greater than the screw the internal diameter decreases and the parts could seize up.